(A) Field of the Invention
The present invention relates to a periodically poled element, and more particularly, to a periodically poled element having a suppressing structure including insulators to suppress the spreading of the inverted domain during the poling process.
(B) Description of the Related Art
Poled structures having poled domains in a ferroelectric single crystal such as lithium niobate (LiNbO3), lithium tantalite (LiTaO3) and potassium titanyl phosphate (KTiOPO4) are widely used in optical fields such as optical communications, optical storage and optical measurement. There are several methods for preparing the poled structure such as the proton-exchanging method, the electron beam-scanning method, the electric voltage application method, etc.
U.S. Pat. No. 6,002,515 discloses a method for manufacturing a polarization inversion part on a ferroelectric crystal substrate. The polarization inversion part is prepared by the steps of applying a voltage in the polarization direction of the ferroelectric crystal substrate to form a polarization inversion part, conducting a heat treatment to reduce the internal electric field generated in the substrate by the voltage applied, and then reinverting polarization in a part of the polarization inversion part by applying a reverse direction voltage against the voltage that was previously applied. In other words, the method for preparing a polarization inversion part disclosed in U.S. Pat. No. 6,002,515 requires applying electric voltage twice.
U.S. Pat. No. 6,353,495 discloses a method for forming an optical waveguide element. The method forms a convex ridge portion having a concave portion on a ferroelectric single crystalline substrate, and a ferroelectric single crystalline film is then formed in the concave portion. A comb-shaped electrode and a uniform electrode are formed on a main surface of the ferroelectric single crystalline substrate, and electric voltage is applied to these two electrodes to form a ferroelectric domain-inverted structure in the film in the concave portion.
However, it becomes very difficult to uniformly control the width of the inverted domain in the ferroelectric substrate as the period of the inverted domain becomes shorter and shorter.